Boku no Hero Academia X Ansatsu Kyoushitsu
by Raja-Ulat
Summary: This is a crossover fanfiction idea/challenge of Boku no Hero Academia and Ansatsu Kyoushitsu. I hope that you will enjoy my contribution. Edit Notes: I did some editing after reading chapter 161 of Boku no Hero Academia. The edits cover how Izuku may have a quirk and Sir Nighteye's involvement with Korosensei. I hope the edits are acceptable.


Boku no Hero Academia and Ansatsu Kyoushitsu Crossover Idea

Disclaimer: This is a crossover fanfiction story/idea of Boku no Hero Academia and Ansatsu Kyoushitsu. I own nothing of the two series other than the crazy idea I have been thinking about for weeks. I hope you enjoy my contribution. Be warned, there will be spoilers from BOTH series.

Setting

The story takes place in the Boku no Hero Academia universe but, unlike canon, this fanfiction universe has assassins. Unlike heroes, vigilantes or villains, assassins are more like hired mercenaries who work for the highest bidder although most of them do have their personal standards. As assassins may at times harm innocent people or cause public disturbances, it is understandable why most heroes do not like them and consider them to be no better than villains. Even villains distrust assassins as there is no guarantee that the assassins would not assassinate them on a later date. Assassins in turn generally dislike heroes who would get in the way of their "work" and distrust villains who are deemed as unprofessional at best. In the current age of heroes, professional assassins are rare due to heroes interfering with their work. However, there is no shortage of powerful individuals who wish to use their talents.

While most assassins do have quirks, they tend to use their quirks in subtle ways so that they can do their "jobs" without attracting too much attention. In fact, most professional assassins have better close quarter combat and/or marksmanship skills than typical heroes or villains due to the need to keep a low profile. Few things are as revealing as a flashy quirk after all. This means that the effectiveness of an assassin is less affected by lacking a combat-orientated quirk or having no quirk at all. With that said, many assassins are all too aware of the fact that skilled and/or powerful heroes and villains are dangerous opponents. Even assassins who can fight well in direct combat would generally prefer avoiding such fights and would not hesitate to fight dirty to win.

It may seem surprising that many assassins refuse to assassinate heroes, especially famous or high-profile heroes, but they do have their reasons. The most important reason is that assassins who kill heroes would be targeted by every hero in the world and few assassins are able to get away from the heroes for long. That is not even counting the amount of public outrage which would result from the assassination of a beloved hero, thereby making the assassin an enemy of an entire country or possibly the entire world. Another reason is that assassins who kill heroes are often abandoned or even betrayed by other assassins who refuse to be associated with "hero-killers" and risk being hunted down as a result.

As for villains, assassins have no problems with killing them if the money is worthwhile. However, dangerously powerful villains are often considered off limits due to being too risky to assassinate. The fact that heroes generally frown upon killing villains, even the truly vile ones, makes most assassins deem them as "too soft" to properly end any "villainous problem".

While most assassins try to keep a low profile, there are a few assassins who are simply too infamous be kept completely secret from the public. The most dangerous assassin in history is known as the Reaper who has killed more people than any other assassin. The most infamous rumour is that the Reaper is actually a quirkless man who is simply very talented in killing people and getting away with it.

Dark Matter Research

In this fanfiction universe, there was top-secret field of biological research which involved infusing living creatures with dark matter in an attempt to generate more of the stuff to serve as a potential power source. When it was discovered that dark matter caused biological mutations, there was interest in using the research to create artificial quirks. It was easy to imagine the military potential of an army of powerful mutated quirk users.

The head of the research, Yanagisawa Kotarou, used the recently-captured Reaper to study the effects of dark matter infusion on a quirkless human being. This research eventually caused the Reaper to mutate into a creature which was later blamed for the destruction of about 70% of the moon's mass and the destruction of the research lab. However, the actual cause of the moon's destruction was actually more complicated.

While there was no denying that the creature was responsible for destroying the research facility and killing many of the staff members, the partial destruction of the moon was actually the result of a study on the effects of natural death for a creature which was infused with dark matter. A mouse was placed inside an automated research facility on the moon and was allowed to die a natural death there. Its death caused dark matter inside it to expand uncontrollably to the point of destroying about 70% of the moon's mass. The only reason why the truth was not revealed to the public was because the creature which was once the Reaper accepted the blame when he reappeared after disappearing for a few days.

The creature was so fast and powerful that the strongest hero at the time, All Might, could not even touch him let alone defeat him. In fact, the creature claimed that he would destroy the entire planet Earth if he was allowed to live until the following year in the month of March. This unfortunately was not entirely false for even if the creature was benevolent, his natural death may lead to the destruction of the whole planet Earth not unlike how the mouse inadvertently destroyed most of the moon after it died naturally. Even if such a scenario had a less than 1% chance of occurring, the hazard was simply too great to be ignored.

With the military, heroes and even infamous assassins and villains desperate to kill the creature before the time limit ends, they were forced to accept a deal he offered to them. He would personally select and train a class of middle-school children to assassinate him before the month of March on the following year. Quite a few heroes did wonder why the creature made such an odd deal, especially a certain "quirked animal hero" named Nezu.

Korosensei

The creature decided to personally hand-pick a number of students from different schools to form what would be known as the Assassination Classroom in an abandoned school building. With his ungodly speed, he was able to quickly choose 30 students from several different schools in a matter of a few days. Oddly enough, he picked students who had problems be it family, social, financial, disciplinary or personal issues. A few examples included a boy named Midoriya Izuku who wanted to be a hero but had no quirk, a boy named Shiota Nagisa who was abused by his mother who wanted him to become her "second life", a boy named Akabane Karma who was Nagisa's friend and was a rebellious problematic student, a girl named Toga Himiko who showed clear mental instability, a girl named Kanzaki Yukiko who came from a strict family that constantly pressured her to excel, a girl named Hazama Kirara whose quirk makes her look and move like an evil spirit, a girl named Uraraka Ochako who came from a poor family, Horibe Itona who was abandoned by his family due to severe financial issues, Shinsou Hitoshi who suffered discrimination due to his brainwashing quirk and even Todoroki Shouto, the abused son of the second highest ranking hero, Endeavour.

While the public failed to understand the reason for the creature's unusual choice of students, his actual intentions were actually more benevolent. When the creature was still the Reaper and was being experimented on for dark matter research, he regularly interacted with a young woman named Yukimura Aguri, the fiancée of Yanagisawa Kotarou. In spite of his initially malevolent intentions, the Reaper came to care about Aguri as a friend and eventually a love interest. Before her tragic death during the destruction of the research facility, she told him her greatest wish was to actually be a teacher who helps those in need of guidance to a brighter future, a wish which she was ultimately unable to achieve due to her engagement with her abusive fiancée. The creature swore that he would help fulfil her wish by training a class of assassins whom he would deem as beloved students worthy of assassinating him.

The creature would eventually be known as Korosensei, a teacher and an assassination target who seemed impossible to successfully assassinate. Besides Korosensei, other members of the "faculty" included Karasuma Tadaomi who was an elite soldier and trainee instructor of the Japanese air force, Irina Jelavic who was formerly an assassin named "Honey Spider" before eventually marrying Tadaomi, a villain known as "Stain", a trio of vigilantes known as Knuckleduster, Pop Step and the Crawler, Aizawa Shouta who was a hero known as Eraserhead and even All Might himself. Needless to say, the Assassination Classroom was most unusual if not downright abnormal. Even so, the students were able to thrive and bond with not only one another but also their teachers, including Korosensei himself.

After almost a whole year of attempted assassinations, examination papers, frustrations, laughter, joy and tears, the students of the Assassination Classroom was finally able to assassinate Korosensei.

Korosensei's Legacy

Not long after Korosensei's successful assassination, the headmaster of U. A. High School, Nezu, offered the students of the Assassination Classroom a chance to attend the high school. One of his reasons was that many of the students in the Assassination Classroom showed promise as potential heroes. Another reason was that he realised that Korosensei was actually a benevolent and dedicated teacher in spite of his terrible deeds as an assassin and wanted to help his students as a fellow educator. Nezu even admitted that he wanted to monitor the students more closely to ensure that they do not lose their way and become actual assassins or villains later in life.

As they were given the chance to stay together in the same high school even though they would attend different departments, the students of the Assassination Classroom unanimously decided to join U. A. High School. The departments offered at U. A. High School included the Department of Heroes, the Department of General Studies, the Department of Support and even the Department of Management.

This is the heroic high school story of Izuku and his friends from the Assassination Classroom. A tale of friendship, heroics, assassinations, romance and sacrifice. A fanfiction which I am challenging YOU to write with the best of your abilities.

Author's Notes:

Kayano Kaede, Yukimura Aguri's younger sister, was not selected by Korosensei but instead became a transfer student after receiving Korosensei's message about him being responsible for her sister's death. Like the canon of Ansatsu Kyoushitsu, she had dark matter tentacles implanted into the back of her neck and wished to kill Korosensei until she learnt about the true circumstances of her sister's death.

Ritsu is included in the Assassination Classroom as an "Autonomously Thinking Fixed Artillery" unit. Like the canon of Ansatsu Kyoushitsu, she became an "online app" for the Assassination Classroom and Hatsume Mei would later try to build an actual robot body for her when the gang attends U. A. Highschool.

Midoriya Izuku would not consider Bakugou Katsuki as a friend due to finally having true friends and comrades from the Assassination Classroom. He may still find Katsuki's determination to win admirable but the lessons in the Assassination Classroom has made him see that the determination to win can only be considered heroic if it is fuelled by noble intentions and is tempered by kindness towards others.

The creation of the dorms occurred earlier than in the canon of Boku no Hero Academia to allow the former students of the Assassination Classroom to be gathered in one location.

Like the canon of Ansatsu Kyoushitsu, Yanagisawa Kotarou tried to kill Korosensei under the codename, 'Shiro'. However, he failed to kill Korosensei and ultimately ended up crippled like his canon counterpart. His dark matter research was terminated by the government but villains may try to replicate his experiments to acquire more powerful quirks. Whether Yanagisawa Kotarou would make a reappearance in the fanfiction story or not is up to you to decide.

All For One did try to kill Korosensei but he ended up defeated and even humiliated by him. As a result, he was determined to acquire more powerful quirks before having a rematch with Korosensei. Whether or not he gets that rematch is up to you to decide but it does mean that he may be even more powerful than canon as a result.

All Might will have to pass on his quirk, One For All. Whether he passes it on to Midoriya Izuku or Togata Mirio is up to you to decide. For that matter, assuming that Midoriya Izuku does not inherit One For All, he may either remain "quirkless" or somehow inherit a quirk similar to Korosensei's. Regardless of whether Izuku remains quirkless or not, he would fight like a quirkless by default unless the situation is dire enough for him to use his "secret weapons" such as a hidden quirk.

I initially had a headcanon that Izuku may have a quirk which is related to changing one's fate but, given the events of chapter 161 in Boku no Hero Academia, I have decided to discredit the idea (the canon explanation is better than my headcanon anyway). Instead, how about a quirk which is pretty much useless on its own unless combined with another quirk, namely the power to inherit a quirk from someone else. This would allow Izuku to somehow gain a weaker version of Korosensei's quirk which lacks the hazards of a dark matter explosion and is not vulnerable to 'Anti-Sensei Weapons'. As for how it would happen, imagine if Izuku took a fatal blow and Korosensei used his abilities to heal Izuku, including using his own slime to fill the gaps in Izuku's reconstructed body tissues. The slime would likely have Korosensei's genetic material mixed in which leads to a possible fusion of quirks.

Assuming that Midoriya Izuku gains a weaker version of Korosensei's quirk, he would gain at least one pair of tentacles on his lower back which can retract themselves to be concealed, produce mucus which can be used to heal injuries, regenerate when damaged, change skin colour (Izuku's tentacles would be green by default to fit with his colour motif but may change colour according to his mood or emotion) and move at the speed of sound (which is much slower than Korosensei's Mach 20 Speed). Other abilities include heightened senses and reflexes to help Midoriya Izuku keep up with the movements of his own tentacles. However, using the tentacles takes a lot of stamina and mental willpower so Midoriya Izuku is unable to use them for prolonged periods without either tiring himself out due to losing stamina or losing self-control due to weakening willpower. Midoriya Izuku can use his heightened senses and reflexes without using his tentacles but they must be activated to be used and will still consume stamina and mental willpower. Although Midoriya Izuku's main body is not unlike that of a quirkless human unless he activates his abilities, it is well-trained thanks to his physical lessons and combat lessons during his time in the Assassination Classroom. He would also possess at least some assassination techniques such as the ones used by Shiota Nagisa, one of the top five most talented assassins in the Assassination Classroom (the other four would be Karma who plays dirty and is a skilled fighter as well, Himiko who is a stab-happy shape-shifter, Shouto who has the strongest quirk in the Assassination Classroom and Izuku whose analytical abilities and talent for mimicking fighting styles are terrifying).

As far as pairings go, I prefer heterosexual relationships by default. Possible candidates include: Midoriya Izuku and Uraraka Ochako, Midoriya Izuku and Toga Himiko (While still a stab-happy individual, this version of Toga Himiko is saner compared to canon.), Midoriya Izuku, Uraraka Ochako and Toga Himiko (Yes, a harem situation is possible.), Horibe Itona and Hatsume Mei (They are both involved in making machines and devices after all.), Shiota Nagisa and Kayano Kaede, Shinsou Hitoshi and Hazama Kirara (I made is such that her quirk makes her look and move like an evil spirit so the two can bond over shared experiences of discrimination.), Akabane Karma and Okuda Manami (Okuda Manami has a chemical creation quirk which allows her to create liquid and gaseous chemicals from her sweat. She cannot create solid objects and she lacks the ability to ignite her own chemicals without harming herself but she is essentially immune to corrosive and toxic chemicals. As for the pairing reason, I did find Akabane Karma's reason for liking to her amusing.) and/or Takebayashi Koutarou and Okuda Manami (They were seen together in the canon epilogue of Ansatsu Kyoushitsu so why not?).

Due to the circumstances of the Assassination Classroom, including possible sharing of personal histories to better understand why Korosensei would select them, the students in the classroom are very protective of one another and would therefore be generally antagonistic towards certain individuals such as Bakugou Katsuki and Endeavour. In fact, they are willing to consider "assassinating" their careers as heroes and the harshest part about the "assassination attempt" would be that they do not even need to lie about the past misdeeds of their victims.

Like the canon of Ansatsu Kyoushitsu, the students of the Assassination Classroom would buy the land to keep their faculty building. It can be used as an outdoor training region.

Due to extensive research into Anti-Sensei weaponry, including lasers and force fields, it is possible to convert them into quirk-nullifying weapons not unlike Eraserhead's quirk. Imagine assassins, including the ones that Korosensei has taught such as Izuku, using these weapons. Not even All For One will be completely safe from them.

With regards to Sir Nighteye, he was obviously involved in predicting Korosensei's future. He accurately predicted Korosensei's death at the hands of his own students. Korosensei considered it as a good ending which Sir Nighteye may consider unusual.

That is all from me for now. I hope that this idea is appealing as a fanfiction story material and appreciate constructive feedback.


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